It is well known to transmit a multiplicity of conversations along a single transmission link by digital techniques. This is achieved by periodic sampling of an analog signal and transmitting the samples in digital form rather than the signals themselves. The Nyquist sampling theorem fixes the minimum sampling rate as twice the highest frequency of the signal being sampled.
One widely used technique for digital information transmission is PCM, i.e. pulse code modulation, whereby each sample is expressed in an 8 bit code. Various other types of coding for digital information transmission are also known and used. All of the conventional digital transmission techniques are characterized in that only a single sample relating to a single conversation is being transmitted at any one time. Therefore, using conventional technology, one can transmit 30 voice and 2 signalling channels over a single communications link.
It is also known to apply two modulations onto a single carrier signal, such as AM and FM for example. These two modulations are totally independent of each other and are not interactive. The use of multiple modulations in this manner can provide simultaneous transmission of only as many different channels as there are different modulations.